In the graphic art field, printing of a printing plate is carried out using a set of color separation films prepared from a color original by use of a lith film. In general, a color proof is prepared from the color separation films for checking errors in a color separation process and necessity of color correction before final printing (actual printing operation). The color proof has been desired to realize high resolving power which enables high reproducibility of a medium image, and to have performances such as high process stability. Further, for obtaining the color proof approximating to actual printed matter, materials used for the actual printed matter such as final print paper (an actual printing paper) as a substrate and a pigment as a colorant are preferably used as materials used for the color proof. As a method for preparing the color proof, a dry method using no developing solution is highly desired.
As the dry method for preparing the color proof, a recording system of directly preparing the color proof from a digital signal has been developed with the recent spread of the electronic system in the preliminary process of printing (prepress field). Such an electronic system is employed for preparing the color proof of particularly high quality, and generally reproduces a halftone dot image of 150 lines/inch. For recording the proof of high image quality from the digital signal, a laser beam which can be modulated by the digital signal and make recording light thin is used as a recording head. Accordingly, it becomes necessary to develop an image-forming material exhibiting high recording sensitivity to the laser beam and showing high resolving power which makes it possible to reproduce highly fine halftone dots.
As an image-forming material used in a transfer image formation method using a laser beam, there is known a heat melt transfer sheet comprising a support having provided thereon a light-heat conversion layer absorbing a laser beam to generate heat and an image formation layer in which a pigment is dispersed in a component such as heat-meltable wax or binder, in this order (Japanese Patent Laid-Open No. 58045/1993). In the image formation method using this image-forming material, heat generated in a laser beam-irradiated region of the light-heat conversion layer melts the image formation layer corresponding to the region to transfer an image onto an image receiving sheet arranged by lamination on the transfer sheet, thereby forming a transferred image on the image receiving sheet.
Further, Japanese Patent Laid-Open No. 219052/1994 discloses a heat transfer sheet comprising a support having provided thereon a light-heat conversion layer containing a light-heat conversion material, a heat release layer having an extremely thin thickness (0.03 μm to 0.3 μm) and an image formation layer containing a colorant, in this order. In this heat transfer sheet, irradiation of a laser beam reduces the bonding force between the image formation layer and the light-heat conversion layer bonded by intervention of the heat release layer to form a highly fine image on an image receiving sheet arranged by lamination on the transfer sheet. In the image formation method using the heat transfer sheet, so-called “ablation” is utilized. Specifically, the heat release layer is partly decomposed to vaporize in a region irradiated with the laser beam, which causes the bonding force between the image formation layer and the light-heat conversion layer in that region to be weakened to transfer the image formation layer of that region onto the image receiving sheet laminated thereon.
These image formation methods have the advantages that final print paper provided with an image receiving layer (adhesive layer) as an image receiving sheet material can be used, and that a multicolor image can be easily obtained by transferring images different in color one after another onto an image receiving sheet. In particular, the image formation method utilizing ablation has the advantage that a highly fine image can be easily obtained, and is useful for preparing a color proof (DDCP: direct digital color proof) or a highly fine mask image.
In the progress of DTP circumstances, an intermediate film taking-out process is removed in the use of CTP (computer to plate), and the need for a proof according to the DDCP system has become strong, rather than the need for proof printing or a proof of the analog system. In recent years, large-sized DDCP having higher quality and stability and excellent in print agreement has been desired.
According to laser heat transfer systems, printing at high resolution is possible, and the systems include (1) a laser sublimation system, (2) a laser ablation system and (3) a laser melt system.
However, all of the above-mentioned respective systems have the problem that the recording halftone dot form is not sharp. The laser sublimation system of (1) has the problems that the approximation to printed matter is insufficient, because a dye is used as a colorant, and that the contour of a halftone dot is blurred, resulting in insufficient resolution, because the colorant is sublimated. On the other hand, the laser ablation system of (2) is good in the approximation to printed matter, because a pigment is used as a colorant, but has the problem that the contour of a halftone dot is blurred, resulting in insufficient resolution, similarly to the sublimation system, because the colorant is scattered. Further, the laser melt system of (3) also has the problem that no clear contour is obtained, because a melt flows.
Furthermore, when the difference in size between the heat transfer sheet and the image receiving sheet is small, a proper vacuum adhesion state can not be maintained in fixing the respective sheets to a recording drum by vacuum suction, so that the degree of vacuum is decreased to deteriorate the transferring properties of the image formation layer. On the other hand, when the difference in size is large, air accumulation is developed between the transfer sheet and the recording drum, resulting in a failure to obtain a good vacuum adhesion state.
In addition, when the difference in size between final paper and the image receiving sheet is small, wrinkles caused by slippage between the samples are liable to be developed. Conversely, when the difference in size is large, there is much waste, resulting in disadvantageous cost.
In the multicolor image-forming material according to the invention, the high process stability has been desired as described above. For example, the image receiving sheet is required to have good conveying properties, and further to have good accumulation properties, because a plurality of recorded cut image receiving sheets need to be accumulated.
In the heat transfer sheet on which a color image is formed, a defect of the image significantly reduces the commercial value. One of the causes of the image defect is that a part of the image formation layer is broken by a scratch, resulting in a failure to transfer that portion of the image, which can cause the defect of the image itself. The reason for this is that a surface of the heat transfer sheet is rubbed with a back face in producing, processing and printing the heat transfer sheet to scratch it. In particular, when the area of the image is large, the probability of occurrence of the image defect increases with the size of the image. Accordingly, in the case of the heat transfer sheet having a large image area, it is required that the image defect is more difficult to develop.
For preventing such an image defect, Japanese Patent Laid-Open No. 270154/1993 describes a method of using a specific polyester-acrylic styrene copolymer as a binder for an image formation layer. Further, there is also used a method of providing a protective layer on an image formation layer, thereby preventing an image defect.
It is possible to decrease the frequency of occurrence of the image defect caused by the scratch to some degree. However, the number of the image defects in one image plane is proportional to the image area, so that when the image area is increased, a problem is practically encountered. Further, the employment of the method of providing the protective layer on the image formation layer for preventing the image defect has raised the problem that the sensitivity of a heat-transferred image is lowered.